Field
The invention relates to an antenna beam of an antenna having a plurality of elements.
Description of the Related Art
A Butler matrix with an antenna array designed for the Butler matrix can be used to form a discrete set of orthogonal beams. Each beam is directed towards a predetermined direction which depends on the phase shifts generated in the Butler matrix. A typical number of beams is four to eight.
In a Butler matrix, phase shifts of different signals are typically formed in passive analog circuits, and hence the phase shifts of signals cannot be changed without changing the whole a Butler matrix. The structure of an antenna array utilizing a Butler matrix is such that the spacing between two successive elements in the antenna array is roughly half a wavelength of the radio frequency radiation in order to form desirable lobes and avoid grating lobes.
However, there are problems involved with the design. The present use of a Butler matrix and an antenna array leaves no room for changing the antenna beam pattern and the configuration cannot flexibly be applied, for example, to omni-directional sites or to a site needing certain directionality.
There have been attempts to solve the problems. Digital beam forming at base band helps with the lobes but adds drastically to the complexity of transmitters and receivers. Furthermore, digital beam forming requires accurate on-line calibration and brings along a lot of additional complexity to the base band processing such as estimation of spatial radio channel characteristics. Hence, there is a need for simpler and still flexible antenna configurations.